The present invention relates to acousto-optic frequency shifters, and particularly to fiber optic frequency shifters utilizing surface acoustic waves or bulk acoustic waves.
Optical frequency shifting is typically based upon the Doppler effect, i.e., the change in frequency due to relative movement between the source and observer. The frequency becomes higher and the wavelength shorter when the source is moving towards the observer, and the frequency becomes lower and the wavelength higher when the source is moving away from the observer.
The Doppler effect has been heretofore used in bulk optics to cause frequency shifts in lightwaves reflected from wave fronts of acoustic waves propagating through optically transparent bulk media. The areas of compression and rarefaction caused by the travelling acoustic wave change the index of refraction in the bulk media so that the incoming light is reflected and/or refracted. Movement of the acoustic wave fronts causes a Doppler shift in the reflected and refracted light, such that the light is shifted in frequency by an amount equal to the frequency of the acoustic wave.
While bulk optic frequency shifters are well known, the development of fiber optic frequency shifters is in its infancy. Recently, a rudimentary fiber optic frequency shifter was disclosed by Nosu et al. in an article entitled "Acousto-Optic Frequency Shifter for Single Mode Fibers", published at the 47th International Conference on Integrated Optics and Optical Fiber Communications in Tokyo, June 27-30, 1983, and in Electronics Letters, Vol. 19, No. 20 (Sept. 29, 1983). A birefringent, single mode fiber was placed in a capillary tube filled with mineral oil, and the capillary tube was placed in the piezoelectric (PZT) cylinder in an off axis position. The PZT cylinder was filled with mineral oil. A standing pressure wave in each cylinder resulted when the cylinders were excited with sinusoidal signals to cause elasto-optic coupling between the polarization modes of the fiber, thereby creating side bands above and below the optical carrier. Each cylinder generated one side band that was in phase and another that was out of phase with the side bands created by the other cylinder, such that one side band was strengthened and the other cancelled.
The Nosu device thus functions by applying pressure to the fiber at discrete intervals along the fiber, specifically at intervals of three-quarters beat length of the fiber. The maximum frequency shift obtainable with the Nosu device is equal to the maximum rate which the PZT cylinders can be practically driven. Further, the amount of power coupled between polarization modes at each coupling--i.e., at each PZT cylinder--is quite small, and thus, to couple a significant amount of power a large number of these PZT cylinders would be required, yielding a quite unwieldy and generally impractical device for use in fiber optic systems.
As discussed in copending patent application Ser. No. 556,636, now U.S. Pat. No. 4,684,215, entitled "Single Mode Fiber Optic Single Side Band Modulator", by Shaw, Youngquist, and Brooks, an alternative approach to frequency shifting is to launch an actual acoustic wave (either a surface wave or bulk wave), for propagation longitudinally along the length of the optical fiber. This approach has the advantage of providing a continuous, virtually infinite, number of coupling points which travel along the length of the fiber, as opposed to the discrete static coupling points at spaced intervals of the Nosu device. Further, actual acoustic waves can be generated at a frequency which is higher than the PZT cylinders of Nosu can be driven, and thus, such actual acoustic wave devices are capable of greater amounts of frequency shift than the Nosu device.
One limitation on acousto optic frequency shifters which utilize actual acoustic waves is that, for maximum coupling between modes, the acoustic wavelength should be equal to the fiber beat length. For present, commercially available high birefringence fibers, the minimum beat length is on the order of 1 mm. An acoustic wavelength of 1 mm corresponds to an acoustic frequency of about 1-5 MHz. Accordingly, there is a need in the art for a fiber optic frequency shifter which utilizes actual acoustic waves, but avoids this limitation such that the maximum possible frequency shift is not restricted by the beat length of the fiber.